1. Field of the Invention
The present invention relates to a comparatively small fuel cell system that is ideal as a power source for portable electronic device and the like.
2. Description of the Related Art
The basic structure of a fuel cell comprises an electrolyte layer sandwiched between a fuel electrode and an air electrode, and a direct current is generated by an electrochemical reaction caused by supplying hydrogen to the fuel electrode and oxygen to the air electrode. Because the electromotive force of a single cell, which represents the smallest unit capable of generating a current, is small, a plurality of cells must be connected in series to enable the fuel cell system to provide adequate voltage to function as the power source for a device. Therefore, the plurality of cells are stacked to form a stack configuration. The level of current that can be drawn from a fuel cell can be increased by increasing the reaction surface area. However, because the power sources for portable electronic device and the like must be kept compact, a structure in which the fuel can be supplied smoothly and efficiently to each cell is necessary to enable a vigorous, large volume reaction to occur within a limited reaction surface area.
The most common method of supplying fuel to each of the cells within the aforementioned stack employs a fuel circulation system in which fuel is supplied in series to the plurality of cells within the stack, and the fuel discharged from the stack is circulated and resupplied to the system. For example, in one known system, unreacted hydrogen within the fuel that has circulated through the plurality of cells within the stack and then been discharged is merged with the fuel supply channel (see Japanese Patent Laid-Open Publication No. 2003-317753). In a fuel cell that uses this type of fuel circulation system, a single pump is sufficient for supplying the fuel. However, in this type of fuel circulation system, because the fuel concentration decreases with increasing distance along the fuel supply route, and the concentration of the resupplied fuel also falls, an auxiliary device such as a fuel concentration regulator must be provided. In comparatively large fuel cells, the pump performance can be increased, and the installation of a large pump and/or an auxiliary device presents no particular problems, but this solution is unsuitable for compact fuel cells designed for use within portable device equipment and the like.
Furthermore, in order to prevent a downstream decrease in the fuel concentration within a fuel cell employing a fuel circulation system, a system has been proposed in which the cross-sectional area of the fuel channel is increased within the downstream cells, thereby increasing the fuel flow rate, and equalizing the current density distribution across each of the cells (see Japanese Patent Laid-Open Publication No. 2002-260710). However, in order to enable the supply of an equal concentration of fuel to each of the plurality of cells that form the stack, this system preferably requires an independent fuel supply to each cell. However, in order to ensure the precise supply of a prescribed quantity of fuel to each cell, a single fuel supply port must be provided for each of the plurality of cells. Unfortunately in compact fuel cells designed for use within portable electronic device, the provision of an individual fuel supply port for each cell is problematic.
One fuel supply system that has been adapted for compact fuel cells is a non-circulating fuel system in which only the precise quantity of fuel required for power generation is supplied to each of the plurality of cells. However, when fuel supply is conducted using such a non-circulating fuel system, in order to ensure the precise supply of a prescribed quantity of fuel to each cell, a single fuel supply port must be provided for each of the plurality of cells, with fuel supply then occurring from each of these supply ports. Possible means for supplying fuel to a plurality of fuel supply ports include providing a fuel supply device at each fuel supply port, or providing a single fuel supply device and then distributing the fuel from this single device to each of the fuel supply ports. Providing an individual fuel supply device at each fuel supply port ensures excellent precision of the fuel supply, but the volume occupied by the fuel supply devices is considerable, making such a system impractical for compact fuel cells. On the other hand, in a system in which a single fuel supply device is provided, and the fuel from this single device is then distributed to each of the fuel supply ports, precise control of the fuel supply volume is impossible.